Physiological sensors such as pulse and cerebral oximeters use light to measure a variety of physiological characteristics in body tissue. The physiological sensor generally includes a sensor pad assembly with a plurality of light sources in optical communication with at least one light detector. When activated, the light sources transmit light at specific wavelengths through the body tissues to the light detector. The amount of light received by the light detector after attenuation by the body tissue is indicative of the physiological characteristic being tested.
As the light source ages, the output of light by the light source decreases. For example, the output by the light source may decrease by 15% after only 100 hours of operation. In some cases, the decrease in output is not significant. For example, pulse oximeters generally do not depend on light source intensity so long as the intensity is high enough to maintain high signal to noise ratios. However, cerebral oximeters are often affected by light source aging because they employ multiple light sources and at least two light detectors to generate an absorption profile of the brain, which is at least in part based upon multiple weighted differences of the optical densities at different wavelengths from different detector locations.
Until now, light source aging was not a problem for cerebral oximetry because cerebral oximeters were only used in operating rooms for approximately 4-8 hours before being discarded or replaced. Light source aging for such a short period of operation is negligible. Today, however, cerebral oximeters may operate for several days in, for instance, an intensive care unit (ICU). Because individual light sources on the sensor have different aging characteristics, light source aging during long-term monitoring can cause erroneous readings of the cerebral saturation if appropriate corrective measures are not taken. Therefore, it is useful to know the age of the light source to determine if the readings are accurate. Accordingly, a monitoring system employing a method of determining light source aging is needed.